Tweak the TextTrackCue "cue index" management

Source/core/html/track/TextTrackCue.h

 /*
  * Copyright (C) 2011 Google Inc.  All rights reserved.
  * Copyright (C) 2012, 2013 Apple Inc.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 52 matching lines @@
 
     double startTime() const { return m_startTime; }
     void setStartTime(double);
 
     double endTime() const { return m_endTime; }
     void setEndTime(double);
 
     bool pauseOnExit() const { return m_pauseOnExit; }
     void setPauseOnExit(bool);
 
-    int cueIndex();
+    unsigned cueIndex();
+    void updateCueIndex(unsigned cueIndex) { m_cueIndex = cueIndex; }
     void invalidateCueIndex();
 
     using EventTarget::dispatchEvent;
     virtual bool dispatchEvent(PassRefPtrWillBeRawPtr<Event>) override;
 
     bool isActive();
     void setIsActive(bool active) { m_isActive = active; }
 
     virtual void updateDisplay(HTMLDivElement& container) = 0;
 
@@ skipping 21 matching lines @@
 protected:
     TextTrackCue(double start, double end);
 
     void cueWillChange();
     virtual void cueDidChange();
 
 private:
     AtomicString m_id;
     double m_startTime;
     double m_endTime;
-    int m_cueIndex;
 
     RawPtrWillBeMember<TextTrack> m_track;
 
+    unsigned m_cueIndex;
+
     bool m_isActive : 1;
     bool m_pauseOnExit : 1;
 };
 
 } // namespace blink
 
 #endif // TextTrackCue_h